Letterpress reverse printing (see Patent Document 1) has recently been developed as a printing method that differs typical relief printing, intaglio printing, planographic printing and screen printing of the prior art. Patent Document 1 discloses a letterpress reverse printing method comprised of a coating step for forming a coated surface by coating a resin onto a silicone resin surface, a step for transferring and removing a resin onto the convex portion of a letterpress printing plate by pressing the letterpress printing plate formed in a prescribed shape against the coated surface, and a transfer step for transferring resin remaining on the coated surface to a substrate, and according thereto, allows the formation of a color filter free of unevenness in the ink thick film and the obtaining of images having highly defined resist agent patterning and a high degree of resin flatness. The application to patterning of printed boards and patterning of electrical circuits is also described in Patent Document 1 (paragraph 0009) as a precision pattern forming method and an alternative to photolithography technology.
Patent Document 2 discloses a method for producing a printed wiring board by letterpress reverse printing in which viscosity in a step for forming a coated surface by coating functional materials, consisting of a conductor material having volume resistivity of 1×10−4 Ω·cm or less, an insulator material having volume resistivity of 1×1010 Ω·cm or more and a resistor material having volume resistivity of 1×10−3 Ω·cm or more, onto release surfaces of these functional materials is adjusted to 50 mPa·s or less. However, there is no specific disclosure of an ink composition required to realize electrical properties enabling formation of fine patterns by letterpress reverse printing as well as practical use.
For example, in order to stably form conductive patterns having a line width of 10 μm or less and thickness of 1 μm by letterpress reverse printing without the occurrence of transfer failures as well as realize functions such as conductivity and insulation required by each of the patterns formed, a specific ink formulation is required for each ink forming the conductor, insulator and resistor. In particular, a conductive ink composition for letterpress reverse printing that allows the formation highly conductive patterns by low-temperature heat treatment at 200° C. or lower requires a specific composition in consideration of electrical properties as well as printability.
Patent Document 3 discloses a precision patterning ink composition that has viscosity and surface energy such that the ink composition is able to form a uniform ink coated film on a blanket when forming a fine, precise printing pattern by letterpress reverse printing, realizes drying properties, adhesiveness and cohesiveness such that a finished printing pattern can be formed on the blanket by the time a printing pattern is formed by contact with the letterpress plate, and is provided with adhesiveness and cohesiveness that enables the ink coated film on the blanket to be transferred completely to a printed substrate, wherein the viscosity of the ink is 5 mPa·s or less, the surface energy is 25 mN/m or less, the composition contains a volatile solvent and a solid that is incompatible with a resin that is soluble in this volatile solvent, and the volatile solvent is a mixture of a rapid-drying solvent and a slow-drying solvent. Although Patent Document 3 provides a detailed disclosure of an ink composition required for forming a precision pattern by letterpress reverse printing, there is no disclosure of, for example, a composition required for imparting superior electrical conductivity to a pattern formed with ink for forming a conductive pattern.    Patent Document 1: Japanese Patent No. 3689536    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2005-57118    Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2005-126608